[llvm] [PGO][SampledInstr] Correct off by 1s and allow 100% sampling (PR #113350)

[via llvm-commits]

xur-llvm wrote:

> For the default fast sampling, we change to use unsigned instead from short with this change. I think that's fine.
> 
> This patch looks good to me with some minor comments. Thanks for working on this.

Some correction: I meant  to say "simple" sampling.

https://github.com/llvm/llvm-project/pull/113350